The most challenging parts were the jenga pieces and the pulley. The jenga pieces kept falling down right before we were going to test the whole design. Sometimes they would accidentally get knocked over when one of us reached across the board. Also they were knocked down when the train or the ball accidentally went down the tunnel when we hit the table. The pulley didn’t work all the time because sometimes the ball didn’t land in the cup because the aim was off. Also it many tries to figure out the perfect length of the string so the pulley actually worked.